Does any change in the molecular structure i.e. change in atomic orientation or any change in atomic level affect the nonlinear refractive index of that molecules ? Can I detect any change in structural conformation through Z-scan technique ?
I am not sure what you mean by third order refractive index. The refractive index of a substance depends on the velocity of light through it, and the contributions of bonds can be represented by their contributions to the relative permittivity, which is effectively the ability of the passing electric field to polarise the bond. The net result is that the local electric field on a bond will usually give the same effect for the same bond, although the overall effect depends also on the number of those bonds per unit volume so ter eare effects if there are density differences. The different sorts of bonds give different effects, and in principle, so does induced electron transfer, however changes due to changes in structure without a significant change in dipole moment or bond nature tend to be small, and after considerable effort before some more modern techniques were formed, refractive index changes did not make a big contribution to structure determination, although they were a standard for assessing the purity of liquids if you knew what the liquid was.
I think that the nonlinear refractive index is probably meant here. This is usually abbreviated as n_2, but relates to the third-order nonlinear susceptibility \chi^(3). If this assumption is true, I would think that the answer can be found in M. Sheik-Bahae et al., IEEE J.QE 27, 1295 (1991). In short, any change in molecular structure, orientation, population of states will affect the absorptive properties of the molecule. These are linked to the refractive properties by the Kramers-Kronig relation. The dominant effect behind nonlinear refraction is typically two-photon absorption. However, if you read the paper carefully then you also find orientation contributions from the Raman effect.
Yes, molecular structure determines the nonlinear refractive index, and therefore changes in it will modify its value. It seems you are thinking in molecular transformations such as cis-trans isomerization, and yes, you can use z-scan to sense the changes. There has been some works on using z-scan for the study of cis-trans transitions in azobenzene compounds, more precisely the Disperse Red 1 pigment
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